This invention relates to rolling cutter drill bits and more particularly to such a drill bit having rolling cutters with cutting inserts or elements mounted within sockets on the cutters, and including the method and apparatus for mounting such inserts on the rolling cutters.
Heretofore, in rolling cutter drill bits in which rolling cone cutters are mounted on the drill bit body for rotation, the cutting elements or inserts have been inserted within openings in the cutter body by a press fit or the combination of a press-fit and shrink fit. Such press fitting of inserts heretofore has been accomplished at a temperature less than around 500.degree. F. as temperatures greater than 500.degree. F. tended to soften the material from which the roller cutter bodies were formed. The materials used for rolling cutters normally have been treated to provide a case hardened surface by carburizing as low-carbon alloy steels are normally utilized for rolling cutters. Relatively low tempering temperatures, such as 300.degree. F. to 500.degree. F., are utilized in obtaining the case hardening of the desired surfaces in the rolling cutters from heat treating in an austenitizing, quenching and tempering cycle.
As an example of such hardening, reference is made to U.S. Pat. No. 4,303,137 dated Dec. 1, 1981 in which ball bearing races of the rolling cutter are carburized, slowly cooled, and annealed for subsequent machining. The cutter is then heat treated by oil quenching from the austenitizing temperatures and then tempered from 400.degree. F. to 500.degree. F. and thus subsequent reheating in the 400-500.degree. F. range has a minimal effect on core and case hardnesses. It is apparent therefore that the temperatures to which rolling cutters have been tempered heretofore have not been above around 500.degree. F. as higher temperatures affect the hardness and strength of the cutter bodies.
However, on the other hand, drag type drill bit with polycrystalline diamond compact (PDC) cutting elements have been formed by heat treated steels which have a microstructural stability at 1000.degree. F. or higher and cutting inserts have been inserted within sockets in the drag type bit body at temperatures as high as 650.degree. F. as disclosed in U.S. Pat. No. 4,360,069, Nov. 1982. A drag type bit does not have journals or bearings which require wear and deformation-resistant surfaces and which also require machining to precise tolerances as necessary for rolling cutters. For these reasons, non-carburizing grades of steel and less complex heat treatments have been used heretofore for drag type bits. In addition, the complexity of typical PDC cutter distributions creates a need for sophisticated fixturing means for press-fitting which is alleviated by using a shrink fit methodology.
Normally for rolling cutter bits, the insert is press-fitted into a mating socket or opening in the cone which is slightly smaller than the insert so that an interference fit is created when the insert is forced into the socket. Under service loading, an insert can become loose and rotate, or actually be dislodged from the cone body. To minimize this problem, an interference of several thousandths of an inch is usually utilized. The precise control of interference levels is known to be critical to bit performance since at low interference the tendency for loosening of the inserts is increased, and at higher interferences, the tendency for cracking of the cutter shell between inserts or at the base of insert holes is increased, leading to cone failure as well as insert loss.
Press fitting of inserts within sockets results in elastic as well as plastic deformation of near surface regions and of asperities or surface irregularities on the outer surface of the insert and the inner contacting surface of the receiving socket. When such plastic deformation occurs during the forcing or pressing of the inserts within the sockets, elastic stresses are redistributed and there is an overall reduction in the effective interference, asperity interaction, and corresponding normal and traction forces acting to retain the insert in its socket. In addition, stress concentrations or high stress areas can also develop in areas where plastic deformation is localized such as at the corner of the insert bottom chamfer or in portions of the hole preferentially loaded due to off-axis pressing or geometrical imperfections. Even when press-fitting into cutter bodies preheated to a temperature as high as 500.degree. F., significant force may be required to press in inserts, causing plastic deformation along the surface contact areas between the outer surface of the inserts and the inner surrounding surface of the receiving sockets. Moreover, the reduction in elastic interference stresses due to localized plasticity along the hole wall cannot be substantially compensated by increased design interference levels because the total integrated cutter strain which affects cutter cracking is principally determined by nominal interferences and is relatively insensitive to local plasticity in the hole. The reduction of effective interferences combined with the development of non-uniformity of stress along the hole depth cause an increased vulnerability of inserts to becoming dislodged from the sockets tending particularly under prolonged periods of use under harsh conditions at high temperatures.
Rock drilling is a very demanding service and the cones or cutters of a rock bit are heat treated to a substantial hardness and certain internal surfaces of the roller cone are carburized to obtain a relatively thick, hard case. Such surfaces include internal bearing surfaces which must be sufficiently hard to avoid undue wear and support the contact loads in demanding drilling service.
Tungsten carbide inserts are normally mounted in a tough, strong material forming the body of the cone or cutter and the inserts are retained in the cutter body by friction and traction maintained by radial compressive stress normal to the interface and hoop tensile stress in the cutter material developed when the insert is pressed into the relatively tough, strong cutter body. The loss of an insert during drilling may cause considerable damage as a result of its hardness to the remainder of the cutting structure by becoming mashed or wedged between two or more of the cones or cutters. This may result in chippage or breaking of other inserts which can result in substantial damage to the bit at times requiring a possible removal and replacement of the bit. An early removal of the bit from the borehole and replacement with a new bit is time consuming and increases the cost of the drilling of the hole.
Additionally, upon the press fitting of inserts within sockets of a roller cone body, fluid may be trapped and compressed within the bottom of the socket which may cause anomalous stress conditions at the hole bottom which can contribute to harmful cracking or increased plasticity.